1. Field of the Invention
This invention relates to a device and method for nondestructive and noncontact detection of faults in a test piece. In particular, the present invention relates to fault detection using measurements of an eddy current or magnetic flux leakage. Furthermore, the invention relates to a device and a method for detecting electrically conductive particles in a liquid flowing in a pipe segment using the eddy currents induced in the particles being detected.
2. Description of Related Art
Conventional nondestructive and noncontact detection of faults in a test piece of a semi-finished metallic product is performed by inducing and measuring eddy currents in the test piece. In doing so, the test piece is exposed to periodic alternating electromagnetic fields through a sinusoidally energized transmitter coil. The resulting eddy currents induced in the test piece induce a periodic electrical signal in a coil arrangement which is used as a probe. This periodic electrical signal has a carrier oscillation according to the transmitter carrier frequency whose amplitude and/or phase is modulated by a fault in the test piece when a fault travels into the sensitive region of the probe. Conventionally, when scanning the test piece, the test piece is moved linearly with respect to the probe; however, arrangements with a rotating probe also known. For example, an eddy current measurement device with a linearly advanced test piece is described in U.S. Pat. No. 5,175,498.
Similarly, electrically conductive particles in a liquid, which flows through the coils, cause eddy current losses. These eddy currents can be determined by measuring the impedance change of the coils. In this way electrically conductive particles in a liquid flowing in a pipe can be detected by means of an inductive coil arrangement. This is especially advantageous for detection of the concentration of metallic particles in the lubricant circuit of a machine in order to draw conclusions about the machine state such as measurements of machine wear.
Another conventional measurement method for nondestructive and noncontact detection of faults in a test piece is magnetic flux leakage measurement (or stray magnetic field measurement), by means of an induction coil with a magnetic yoke, which magnetizes the test piece resulting in a magnetic flux leakage produced by the test piece. The magnetic flux is measured by means of a suitable sensor. Faults in the test piece are detected based on their effects on the magnetic flux leakage. One example of this flux leakage measurement can be found in U.S. Pat. No. 4,445,088.
In eddy current measurement devices containing probes which rotate around the periphery of the test piece, measuring the distance between the probe head and test piece is performed in order to correct the measurement with respect to the distance because the distance fluctuates during the course of one revolution. The measurement of the distance is performed because of decentering or asymmetry of the cross section of the test piece occurs during one revolution. One example of this arrangement can be found in German Patent Application No. 40 03 330 A1.
International Patent Application Publication WO 2006/007826 A1 discloses an eddy current measurement device with a digital front end, such that the A/D converter stage is triggered with a n-th integral fraction of the frequency of the carrier oscillation, where n is selected depending on the fault frequency, i.e., the quotient of the relative velocity between the test piece and probe and the effective width of the probe.
U.S. Pat. No. 4,209,744 describes an eddy current measurement device which has a test means which simulates signals that are typical of faults in a test piece in order to perform fundamental checking of the electronics. However, only a single amplitude and a defined primary fault frequency can be simulated. Even if the simulated fault signal were provided with variations, all the electronics cannot be tested. Furthermore, such a simulated fault signal cannot be traced to a certified reference element without dismounting all the electronics and sending them to a laboratory.
International Patent Application Publication WO No. 01/22075 A2 describes an eddy current measurement device within the framework of self-calibration of the system. The intensity of the signal originates from a segment of a test piece which does not contain a fault.
GB Patent Application No. 2 192 064 describes an inductive test device where the device is detuned to simulate a fault by a self-test means and by connecting a LED.